The present invention relates to a novel copolymer, a rheology control mixture and coating compositions containing the copolymer, and methods of preparing the copolymer and using the copolymer in a coating composition.
An ongoing trend in the automotive industry has been to develop coating systems which provide smooth, glossy, mirror-like finishes in both solid colors and the currently popular metallic colors. Recently, there has also been an effort in the automotive industry to reduce atmospheric pollution due to volatile solvents emitted during the painting process. One approach to this end has been to develop "high solids" coating compositions containing low molecular weight polymeric binders. Unfortunately, many of the high solids coating compositions have a tendency to sag on application and when baked. Such sagging adversely affects the appearance properties of both pigmented and unpigmented coating compositions. The adverse effects of sagging on the appearance properties of cured films are particularly severe when the high solids coating compositions contain metallic flake pigments. The metallic coating compositions also tend to have poor pattern control; that is, orientation of the metallic flake pigment, which results in poor appearance. Control of pigment orientation in metallic high solids coatings provides a lustrous shiny appearance ("brightness of face") in the cured films along with excellent flop. By flop is meant the visual change in brightness or lightness of the metallic coating as the viewing angle changes; that is, a change from 90 to 180 degrees. The greater the contrast from light to dark appearance with respect to viewing angle, the better the flop. Flop is a desirable appearance property because it accentuates the contours of a curved surface such as on an automobile body.
In an attempt to alleviate sag problems, materials such as pyrogenic or fumed silica have been added to high solids coating compositions. Also as disclosed in EP 0457939 B1 the hydrophobic type silica can be used with a nonionic fluorocarbon polymer bridging agent. However, such materials may not provide adequate pattern control in metallic coating compositions and may decrease the distinctness of image in clear coatings. Distinctness of image is the mirror-like quality of a coating system and is directly affected by the smoothness of the coating.
It would be desirable to provide a low-cost rheology control additive for use in conventional coating compositions which provides acceptable sag control and pigment orientation without adversely affecting other appearance properties, for example, distinctiveness of image. It is also desirable to have such a coating composition by inclusion of or modification by as few components as possible for instance by the addition of one or two additional or modified components that cooperate with each other. It is also desirable to provide a component that cooperates with existing rheology control additives or forms a mixture with rheology control additives to yield a coating composition which can be used as either a pigmented or unpigmented coating in a color plus clear composite coating.